The present invention relates to an electron cyclotron resonance ion source. It has numerous applications, as a function of the different values of the kinetic energy range of the extracted ions and can be used in thin layer sputtering, microetching, ion implantation, heating by fast neutrons the plasma of fusion reactors, tandem accelerators, synchrocyclotrons, etc.
In electron cyclotron resonance ion sources, the ions are formed by strongly ionizing a gas or a vapour of a solid contained in an ultra-high frequency cavity, as a result of the combined action of a high frequency electromagnetic field established in the cavity and a resultant magnetic field prevailing in said cavity. The magnetic field also has an amplitude B.sub.r satisfying the electron cyclotron resonance condition B.sub.r =f.multidot.2.pi.(m/e) in which m is the mass of the electrons, e its charge and f the frequency of the electromagnetic field. This resonance makes it possible to strongly accelerate the electrons formed which, by impact on the neutral atoms of the gas or vapour, make it possible to strongly ionize the latter.
The operation of a cyclotron resonance source has more particularly been described in U.S. Pat. No. 4,417,178 filed in the name of the Applicant.
Hitherto, the constructions of electron cyclotron resonance ion sources, such as for example that described by R. Geller, C. Jacquot and P. Sermet in the "Proceedings of the Symposium on ion sources and formation of ion beams", Berckeley (October 1974) and F. Bourg, R. Geller, B. Jacquot, T. Lamy, M. Pontonnier and J. C. Rocco in "Nuclear Instruments and Methods", North-Holland Publishing Company 196, 1982, pp. 325-329 are based on the establishment of a confinement of the plasma with the aid of a magnetic mirror configuration. In the construction according to the first reference, the magnetic mirror configuration is obtained by means of three groups of coils.
FIG. 1 is a graph showing the curve of the magnetic field as a function of the distance along the central axis of the ion source according to the prior art by superimposing with a diagrammatic representation of the location of the main elements constituting this source. As shown in FIG. 1, the curve of the magnetic field 1 supplied by the coils has two maxima at the locations of the first group 2 and of the third group 4 of coils and a minimum between these two maxima at the location of the second group 3 of the coils, said latter group having a counter-field supply.
The maximum values are higher than the magnetic induction value B.sub.r corresponding to cyclotron resonance, resonance being reached between the two maxima. Thus, the plasma is created and confined in the area of the ion source located between said magnetic field maxima. The maximum and minimum values of the magnetic induction of said ion source are in this case 4200 and 3200 Gauss respectively. Electron cyclotron resonance takes place at 3600 Gauss, the frequency of the injected high frequency wave being fixed at approximately 10 GHz.
The ions created in the plasma are finally extracted by an extraction system 5, constituted by electrodes, which are located downstream of the second maximum of the magnetic field. Moreover, if as in the example described hereinbefore, the ion extraction system is positioned downstream of the second magnetic field maximum and if the latter is reduced, the ion current emitted by the source is reduced proportionately.
To obtain an intense ion current, the ions are consequently extracted in a magnetic field of the same order of magnitude as the cyclotron resonance field. If the ion beam is emitted in the magnetic field produced by the group of coils and if the magnetic field is suddenly eliminated downstream of the second coil of the ion source, the ions take up transverse energy and the ion beam diverges, i.e. its optical qualities are destroyed. This effect is described in the Bush theorum.
In order to retain the optical qualities of the beam downstream of the ion source, it is then necessary to keep the magnetic field constant in all the sliding space of the ion beam up to the location of its application or the transformation of the ions into neutral particles. For the example described hereinbefore, the field to be kept constant corresponds to an induction of approximately 3600 Gauss, whilst the electrical energy consumed by the coils 6 creating said magnetic field is approximately 1 megawatt.
In the case of the use of low energy ions (below 1 keV), the extraction system does not make it possible to extract the high densities. In order to increase the latter, it is possible to compress the ion beam downstream of the ion source. The magnetic field must be increased proportionately in order to compress the ion beam. Thus, the increase of the ion current density is limited by technical problems which occur with respect to the production of magnetic fields of this order of magnitude.
In summarizing, the prior art ion sources suffer from the disadvantages of a very high energy consumption of the magnetic configuration whilst the increase in the density of the low kinetic energy ion current is problemmatical due to the need for a high magnetic field.